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US dolphin deaths set to rise as migration begins

By Jeff Hecht

The US east coast has seen a tenfold increase

(Image: L.Todd/AP/PA)

COOLER conditions are returning to the US Atlantic coast – a signal for the local bottlenose dolphin population to migrate south. This year, the animals have an unwelcome passenger&colon; a lethal virus that has already killed hundreds and will now spread with them. New insights suggest a web of species keeps this and related viruses going around in cyclical epidemics.

Well over 400 dolphins have died from New York to North Carolina this year – a death rate 10 times as high as normal. Genetic tests by the US National Oceanic and Atmospheric Administration last month confirmed that the cause of the deaths was cetacean morbillivirus, a cousin of the virus that causes measles in people. The virus survives at low levels in pilot whales, which are thought to have passed it on to the dolphins.

In the weeks ahead, NOAA will monitor bottlenose dolphins around the south-eastern US states, says Erin Fougères, a marine mammal biologist and stranding program administrator for NOAA Fisheries. The agency fears that the disease will spread to non-migratory bottlenose dolphins. Fougères adds that the epidemic could reach as far south as Florida.

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The problem is one of population size. Species that number in the hundreds of thousands, such as pilot whales, can keep the virus in circulation without harming the population as a whole. Most individuals are exposed when they are young, and immunised. Species with smaller populations, like bottlenose dolphins, cannot sustain the virus. As a result, it passes through them in epidemic waves. Mothers that survive an epidemic pass antibodies on to their young. But the antibodies only stick around for a few months, according to a study earlier this year by Ab Osterhaus of the Erasmus Medical Center in Rotterdam, the Netherlands. So immunity fades with time (Emerging Microbes & Infections, doi.org/ns4).

In this way, some 22,000 harbour seals in Dutch waters died in an epidemic of seal morbillivirus in the late 1980s. Osterhaus showed that survivors retained immunity initially, but it faded as the years passed, and over 30,000 died in another epidemic in 2002. He has monitored the seals since and says immunity is now very low. He predicts that the seals are at high risk of a further outbreak any time in the next few years. While the dolphins are infected by pilot whales, he believes harbour seals get the virus from harp seals, which number in the millions in the Arctic.

Epidemics are probably also cyclical in bottlenose dolphins. An outbreak killed 800 along the US east coast in the late 1980s, though the real toll was probably much higher as many die at sea. There are now 30,000 bottlenose dolphins in the north Atlantic, and 25 years after the last morbillivirus epidemic, few retain immunity, according to a study led by Juan Antonio Raga of the University of Valencia in Spain.

Bottlenose dolphins and harbour seals can withstand occasional outbreaks – but the same is not true for all species. There are fewer than 1000 Mediterranean monk seals alive today. In 1997, the largest population – off the west African coast – was reduced by half. Osterhaus found morbillivirus in some of the dead seals. Another outbreak could be very bad news for the species.